The present invention relates to a PWM (pulse width modulation) pulse generator for generating a PWM pulse used to control a PWM inverter.
The PWM inverter is widely used to control the speed of an induction motor. The firing of the PWM inverter is controlled by the PWM pulse produced by comparing a sine wave voltage signal (modulating wave) with a triangular signal (carrier). The amplitude and frequency of the sine wave voltage signal are determined such that a fundamental wave component of an output voltage of the PWM inverter corresponds to the amplitude and frequency required by the induction motor.
Recently, a vector control method has been proposed and put into practice in which a primary current of the induction motor is reduced to a component in the same direction to a magnetic flux axis (exciting current component) and a component orthogonal thereto (torque current component) and these components are individually controlled. By employing the vector control method, a high precision torque control of the induction motor is attained as is done in a DC motor. The vector control method is disclosed in U.S. Pat. No. 3,824,437 (corresponding to West German Patent Laid-Open (DOS) No. 1,941,312) and Japanese Patent Application Laid-Open No. 11125/76.
In the vector control method, the amplitude and the frequency of the primary current as well as the phase thereof are momentarily changed. When the vector control method is employed to drive the induction motor by the PWM inverter, the amplitude, the frequency and the phase of the modulating wave are changed. On the other hand, the carrier is controlled such that it is synchronized with each period of the modulating wave. Accordingly, even if the phase of the modulating wave is changed, the carrier is not changed until the end of the period. Thus, the PWM pulse has a waveform which causes the output voltage (fundamental wave component) of the PWM inverter to be non-sinusoidal. As a result, a high precision vector control is not attained.
The vector control may be effected by a digital control by using a microprocessor. However, when the carrier is triangular wave, it has an inflection point which results in discontinuity. Accordingly, a software processing is hard to attain.